1. Field of the Invention
The present invention relates to apparatus and methods for photo-electrically detecting (i.e., reading) high density information from optical storage media such as optical discs, and more specifically to such apparatus and methods particularly adapted to detect phase differences imparted to reading light by recorded tracks on the storage media.
2. Description of the Prior Art
One currently preferred approach to high density optical information storage is to record tracks of minute (usually on the order of a micron or less in size) marks on the storage medium. The most popular format is for such storage medium to be of disc shape and for the tracks to be spiral or concentric so that optical scanning can be effected by rotating the disc relative to a small focused spot of read-light (e.g., from a laser). However, there are various other equivalent formats to disc shape and reference herein to optical discs is intended to include all such equivalent optical storage medium.
In general, there are two kinds of systems for storing and retrieving information on optical discs, distinguished primarily by the construction of the recording medium. These can be termed amplitude systems (wherein a read signal is obtained primarily from the difference in amplitude of read light reflected from marked and non-marked portions of the optical disc) and phase systems (wherein a read signal is obtained primarily from the difference in phase of light reflected from marked and non-marked portions of the optical disc). Although the present invention can be utilized with advantage in reading both amplitude and phase systems, it is particularly useful for reading optical discs of the phase system kind. The record tracks of this kind of optical disc are usually comprised of discrete pits or indentations of predetermined depth below the intervening non-pitted track portions. Land portions, of height equal to the non-pitted track portions are located between adjacent tracks to prevent cross-talk. Heretofore, read-out of phase system optical discs has most commonly been practiced by one of two approaches, which can be termed "central aperture" and "split-detector".
In central aperture reading of optical discs, a reading spot, of effective width greater than the track width, is located in a centered relation along a track so that pitted and non-pitted portions of the track scan therepast at a rate determined by the disc rotation speed. The photo-electric detector is located in the center (i.e., on the axis) of the light beam reflected from the scanned disc and receives all such reflected light that is collected by the read-out objective (e.g., by focusing such collected light onto the detector). The pitted portions of the track are of a predetermined optical depth (.lambda./4 with respect to the reading light wavelength .pi.) so that a .lambda./2 or .pi. phase difference will be imparted to light rays respectively reflected from pitted and non-pitted regions. The peak-to-peak detector signal for the central aperture system is obtained between a maximum constructive interference condition (when light passing to the photodetector is maximally from non-pitted track portions and adjacent lands) and the maximum destructive interference condition (when light passing to the photodetector contains its largest proportion of light from pitted portions that destructively interfers with light from adjacent lands). This technique is described in more detail in an article entitled The Optical Scanning System of the Philips "VLP" Record Player, at pages 186-189 of Volume 33 of the Philips Technical Review, No. 7, 1973.
One example of the "split-detection" approach for reading phase difference information from optical discs is disclosed in U.S. Pat. No. 4,065,786. That patent indicates an objective to utilize a read-spot of lesser cross track dimension (width) than the central aperture approach, so as to minimize the likelihood of cross-talk between adjacent information tracks. In order to utilize such a reduced read-spot width and still maintain the high resolution obtainable with the central aperture detection, the patent suggests that two light detectors be located in a non-centered position with respect to the central axis of the light beam returned from the interrogated track on the disc. Specifically, the light detectors are located respectively to respond to the light intensity in two overlap zones of the light retroreflected from the disc, viz., the overlaps of the zero diffraction order beams returned from the disc with each of the + and - first diffraction order beams returned from the disc. The outputs of the two light detectors are combined (subtracted) to double the amplitude of the recovered signal.
Besides reducing the likelihood of cross-talk, the split detection approach provides an additional potential advantage over the central aperture detection approach. Specifically, split detection provides a maximum signal when the phase of light reflected from pitted and non-pitted portions of the disc differs by .pi./2, rather than .pi. as in central aperture detection. Thus, in systems in which phase shift depends on pit depth, optical depths of only .lambda./8 can be used with split detection (in contrast to the .lambda./4 optical pit depths that optimize central aperture detection); and the use of "shallower" pits can be very advantageous for real-time recording approaches where desired pit depth dictates the laser recording power and/or speed of operation.
However, split detection is not without disadvantages. As mentioned, that approach requires precise placement of two detectors with respect to the two overlap zones between zero and first diffraction order light beams. In addition, the detector must be placed in a plane in space in which such overlap zones are accessible, e.g., at a plane that is image conjugate to the exit pupil of the read-out objective, and only light in the overlap regions contributes to the signal recovered from the disc. In general, the amplitudes of the interfering zero and first order beams are not equal in the regions where these beams overlap; and the depth of modulation of the signal resulting from the mutual interference of those beams is limited by this fact. Moreover, since the signal generated by the zero/plus first order overlap is 180.degree. out of phase with that generated by the zero/minus first order overlap, two detectors must be used to obtain maximum signal depth of modulation; and these detectors cannot be at the focus of a field lens. This situation complicates system alignement.